Currently, dynamic Time Division Duplex (TDD) configuration has been introduced in Long Term Evolution (LTE) system. The dynamic TDD configuration means that the TDD configuration of a radio base station may vary depending on actual traffic requirements. For example, if more traffic is required in the downlink, more subframes can be allocated to the downlink. Otherwise, if more traffic is required in the uplink, more subframes can be allocated to the uplink.
FIG. 1 illustrates an example of a dynamic TDD configuration having a primary TDD configuration and a secondary TDD configuration. As shown in FIG. 1, the primary TDD configuration has more uplink subframes, while the secondary TDD configuration has more downlink subframes. A downlink subframe is illustrated by a white box with a down arrow whereas an uplink subframe is illustrated by a gray box with an up arrow. Those subframes which are configured with the same direction in the primary and secondary TDD configurations are not allowed to change their directions and those subframes which are configured with different directions between the primary and secondary TDD configurations can be dynamically configured for either downlink or uplink data transmission. In the following description, the former subframes are referred to normal uplink/downlink subframes while the latter subframes are referred to as flexible subframes.
Although the dynamic TDD configuration is suitable to meet traffic requirements, it may be problematic for a UE measuring a downlink quality (e.g., Reference Signal Receiving Power (RSRP) and/or Reference Signal Receiving Quality (RSRQ)) of a neighboring BS, which is intended to be selected as a target BS, during a handover preparation period. In a handover preparation, the UE may assume that the TDD configuration of a neighboring BS is the same as that of its serving BS and then perform measurements on the neighboring BS's downlink quality on several downlink subframes in the TDD configuration of its serving BS. However, the actual TDD configuration of the neighboring BS may be different due to the dynamic TDD configuration.
FIG. 2 exemplifies TDD configurations of a serving BS and a neighboring BS of a UE. The TDD configuration of the serving BS is DSUDDDDDDD and the TDD configuration of the neighboring or target BS is DSUUUDSUUU. “D” denotes a downlink subframe and is illustrated by a white box with a down arrow in FIG. 2. “U” denotes an uplink subframe and is illustrated by a gray box with an up arrow. “S” denotes a shared subframe and is illustrated by a white-gray mixed box. The UE served by the serving BS may measure downlink quality of the target BS at subframes 0, 1, 3, 4, 5, 6, 7, 8, 9 as shown in FIG. 2, even though subframes 3, 4, 7, 8, 9 are uplink subframes for the neighboring or target BS.
For example, when a UE moves from its serving cell to a neighboring cell with less normal downlink subframes (i.e., moving from being served by its serving BS to being served by a neighboring BS), the downlink quality (e.g., RSRP and/or RSRQ) may be underestimated if the UE assumes that the same normal downlink subframes are configured in the neighboring cell as in the serving cell and generates the downlink quality for reporting to the serving BS by filtering measurements for all these subframes in accordance with wireless handover techniques. This means that the UE may miss the proper time to trigger the handover procedure and that there is a high dropping probability.
Downlink quality underestimate generally occurs in the following two scenarios:                When a UE moves from a macro cell to a pico cell, the pico cell may be configured with less normal downlink subframes since some subframes are configured as flexible subframes;        The same TDD configuration for all pico cells may not be suitable because downlink and uplink traffic loads in pico cells may be different. So, it is allowed to configure different primary TDD configurations for pico cells. In such a case, a UE moves from a pico cell with more normal downlink subframes to another pico cell with less normal downlink subframes.        
Still take FIG. 2 as an example and assume the TDD configuration of the serving BS is DSUDDDDDDD and the TDD configuration of the neighboring or target station is DSUUUDSUUU. Since subframes 3, 4, 7, 8, 9 are uplink subframes for the neighboring BS, RSRP/RSRQ measured by the UE in these five subframes are quite low. However, the UE still takes them into account when generating the final RSRP/RSRQ measurement report, which means e.g. a rough 3.5 dB underestimate of the downlink quality of the neighboring BS. This may have a clear negative impact on making handover decision.